1. Field of the Invention
The present invention relates to a power transmission system, such as for an electric automobile using an electric motor as a drive power source or an electric automobile using a fuel engine (an ordinary engine making use of an explosion of fuel) and an electric motor both as a drive power source, and to an operation method for the same.
2. Description of the Related Art
There is disclosed in Japanese Patent Application Laid-Open Publication No. 9-226394 a drive system 2001 for electric automobiles shown in FIG. 22.
The drive system 2001 for electric automobiles includes an electric motor 2003, a reduction gear set 2005, and a differential apparatus 2007.
The electric motor 2003 is driven by a vehicle-mounted battery to produce drive power, which is decreased in speed by the reduction gear set 2005 within a range of the number of revolutions of a traveling wheel and increased in torque, and resultant drive power is distributed via the differential apparatus 2007 to wheels.
In the drive system 2001, as shown in FIG. 22, the electric motor 2003 and the reduction gear set 2005 as well as this reduction gear set 2005 and the differential apparatus 2007 are connected directly with no clutch disposed on the way.
On the other hand, FIG. 23 shows an example of a power transmission system for a four-wheel driven vehicle having an ordinary engine as a drive power source.
This power transmission system includes a transversely arranged engine 2101, a transmission 2103, a belt-driving transfer 2105, a front differential 2107 (as a differential apparatus for distributing drive power from the engine 2101 to left and right front wheels), front axles 2109 and 2111, left and right front wheels 2113 and 2115, a rear-wheel end propeller shaft 2117, a coupling 2118, a rear differential 2119, rear axles 2121 and 2123, left and right rear wheels 2125 and 2127.
The drive power from the engine 2101 is transmitted from an output gear 2129 of the transmission 2103 via a ring gear 2131 to a differential case 2133, to be distributed from the front differential 2107 via the front axles 2109 and 2111 to the left and right front wheels 2113 and 2115, or to be transmitted via the differential case 2133, the transfer 2105, and the propeller shaft 2117 to the rear wheel end.
In this arrangement, the coupling 2118 disposed in a power transmission subsystem at the rear wheel end is employed for control of torque transmission to the rear wheels 2125 and 2127.
For example, when the coupling 2118 is connected, drive power from the engine 2101 is distributed from the rear differential 2119 via the rear axles 2121 and 2123 to the left and right rear wheels 2125 and 2127, rendering the vehicle four-wheel driven.
When the coupling 2118 is disconnected, the rear differential 2119 and subsequent elements of the rear wheel end subsystem are cut off, rendering the vehicle two-wheel driven.
In the conventional electric automobiles, an electric motor is connected directly to wheels, and even in a case of interruption of power supply to the electric motor, such as when travelling by inertia, the electric motor is forced to rotate together with wheel rotation, with burdens on the electric motor.
For example, if the electric motor is a brush type like a DC motor, brushes receive a great influence on the durability, resulting in increased maintenance costs, such as by increase in number of services for brush replacement.
Further, when caused to rotate by wheels, the electric motor serves as a generator, producing electromotive forces (emf). As the rotation is increased in speed by the reduction gear set, there is produced a greater emf, resulting in burdens, such as on a battery or alternator (not shown) or on circuit elements such as a regulator constituted as an integrated circuit, causing a reduced durability.